JL, you haven't given quite enough details to answer your question. If you actually mean "A/B" then, as the other replies indicated, the B speakers just play the same program material as the A speakers, and you can't accomplish what you propose. On the other hand, some Onkyos(you haven't specified your model), have provisions for a "Zone 2" which allows playing a different program on the Zone 2 speakers and controlling their volume(e.g. the 703). If the back surround amplifier channels are used for Zone 2, then of course you couldn't use them simultaneously for back surrounds in a 7.1 setup. Study your manual.
